This invention relates to an apparatus and method for testing the mechanical properties of a sample and particularly of a sample which is either relatively small in size or of a material which has a very steep force versus deformation characteristic.
Standard equipment is available for testing the stress and strain characteristics of a sample. In such devices a force or stress is applied to the sample and the deformation or strain is then detected so that calculations can be made on the properties of the material forming the sample depending upon the results of various test parameters.
For many years fundamental tests have been available to measure the properties that are inherent to the material under test and do not depend upon the geometry of a tested sample, the conditions of loading or the apparatus. Examples of these properties are modulus of elasticity, poisson's ratio, relaxation and retardation time, sheer modulus, etc.. The fundamental tests as applied to any sample material may be classified into two essentially different groups; those conducted under conditions of static or quasi-static loading, and those conducted under dynamic conditions.
Machines have been available for many years to carry out such testing often known as a universal testing machine such as those manufactured by Instron or Chatillon.
There are a number of tests developed which may be used to study visco-elastic materials such as biological materials and determine their relations between stress, strain and time for given type of deformation and a given type of loading pattern. The most important tests include stress-strain, creep-recovery, stress-relaxation and dynamic tests.
In creep-recovery tests, the load is suddenly applied and held constant for a given period of time and then removed. In this test the deformation is measured as a function of time. Rheological models are available to calculate constant characteristics for the material from these tests.
In a further series of tests, the deformation relative to time can be determined for a plurality of cycles of repeated cycles of loading and unloading of the sample.
In all these tests it is necessary to apply to the sample a predetermined maximum force. For creep-recovery tests the rate of loading must be relatively rapid since the formulae assume that the loading is applied substantially instantaneously.
One machine which is available for these tests is known as a Universal Tester Model ET11OO manufactured by John Chattilon & Sons Inc. of New York. This machine includes an upper transverse bar which is fixed to a structural frame and a lower transverse bar which is movable in a vertical direction under very accurate control from a pair of lead screws driven by a constant velocity motor. A first and a second sample engagement plate are mounted on the upper and lower bars respectively and adjusted so that they lie parallel and at right angles to the direction of movement of the bar. The sample is then positioned between the plates so that it is compressed by upward movement of the bar. A load cell can be positioned between the upper plate and the upper bar to determine the force applied to the sample. The deformation of the sample is measured by detecting the location of the movable lower bar through the control system moving the lower bar.
This device is well known and is a standard machine available in many testing laboratories for carrying out the standard tests. The machine is entirely satisfactory and highly accurate for many operations where the sample is relatively large and where the sample is relatively resilient so that the deformation of the sample is relatively large in comparison with the force applied.
The above prior art machine uses a lead screw drive system for the accurate control of the bar movement. Other prior art machines for example manufactured by Instron use a hydraulic drive system for this accurate control. The present invention is concerned with both types of machine.
Up till now however the standard machine has been entirely unsuitable for carrying out tests on very small samples such as those less than five millimeters in diameter or in carrying out tests on samples in which the deformation is very small thus causing the force to increase very rapidly.
The problem with the machine in its standard form is that it is necessary for the tests to set a maximum force or predetermined force to which the machine will move and will then halt. Unfortunately when the deformation involved is very small, the feedback between the load cell and the motor is insufficiently accurate to ensure that the machine stops at the predetermined force. This predetermined force can in such cases vary significantly by as much ten or twenty percent leading of course to completely inaccurate results. The machine and the standard tests defined for the machine provide no solution to this problem and accordingly the machine has generally been considered to be unsatisfactory for testing of small samples or samples where the force increases dramatically for small deformation so that the movement of the device is insufficiently controlled to accurately limit the maximum force to be applied. Thus it has been impossible to perform creep-recovery tests with this machine up till now.
An alternative machine is available which uses a beam along which a load moves at a constant velocity so that the force applied to a sample upon which the beam rests increases at a constant rate provided the load moves along the beam at a constant rate. In theory this machine should allow very accurate control of the maximum force. This machine also allows the increase of that force up to the maximum at a constant rate.
However, in practice the load must be halted at a particular location along the beam and of course the momentum of the load causes a dynamic impact at the point of halting of the load and this interferes with the accurate testing of the sample.
At the present time, therefore, there is no machine available for carrying out these tests on the very small samples.